Blood is commonly separated into its components by centrifugation. This may be done for many reasons, one of which is to obtain a selected component for use in treating a patient. For example, as disclosed in U.S. Pat. No. 6,398,972 (Blasetti) blood is separated into components by centrifugation to obtain platelet rich plasma, which is used in the autologous treatment of a patient. In this system, the whole blood is placed in one chamber of a processing unit, and the platelets and plasma are decanted to a second chamber after a first centrifugal spin. Then, subjecting the processing unit to a second centrifugal spin separates the platelets from the plasma. It is important to ensure that the platelets, as well as other desired components, such as white blood cells, are separated from the red blood cells in the first spin and decanted with the plasma while the red blood cells remain in the first chamber.
Blood, a physiological fluid, comprises a suspension of particles in a fluid, and includes principally plasma, red blood cells, platelets, and white cells, as well as many others. The density of plasma is generally about 1.020, and the density of platelets is about 1.040. The density of red blood cells varies between 1.07 to 1.09 depending on the age of the cell and other factors. The density of white cells lies between that of platelets and red cells. In practice, a “layer” will not be of purely one type of cell and may contain several types of cells. Thus, it is the usual practice to refer to the average density of a layer that is principally of one cell type but contains other cell types. For example, after centrifugation, the “plasma layer” will be principally plasma but will contain other cells, such as platelets and red blood cells, which raise the average density of the layer by an amount that depends on their proportions in the layer.
Another known system (U.S. Pat. No. 7,077,273) maintains separation between a supernatant and red blood cells by use of a separating element in the container of a physiological fluid, such as blood, subjected to centrifugation to assist in ensuring that the red blood cells remain in a chamber as the plasma, platelets and other components are transferred, as by decanting, to a second chamber. In early embodiments, a separating element was fixed in position in the first chamber at the expected location of the boundary between the red blood cells and the other components. That structure was found to be less than optimal because the actual location of the boundary between plasma and red blood cells is a function of several variables, such as the hematocrit of whole blood (i.e., the percentage of blood that comprises red blood cells), and the duration and G-force of the centrifugation. Yet another factor that affects the actual location of the boundary and the average density of the layers is the sedimentation rate of the components, which is affected by numerous factors, including the tonicity of the anticoagulant or other solutions that crenate, or swell, the cells, the age of the red blood cells (older cells are more dense), the age of platelet (younger cells are more dense), the size of the cells (large white blood cells sediment faster than smaller cells of equal density), the rouleau of red cells, and the viscosity of the plasma.
Separating elements designed to float in the physiological fluid are known, these elements being configured to float at or near the boundary between red blood cells and the desired supernatant elements. One such floating element shown in U.S. Pat. No. 7,077,273 is in the form of a disk designed, by its configuration and density, to position a surface just below the interface between red blood cells and the buffy coat (white blood cells and platelets). In that design, however, the focus is on the density of the red blood cell layer in the region of the interface with the buffy coat. While this design has proven successful, the desire for increases in the proportion of the target components to be recovered from the fluids (e.g., platelets and/or stem cells) that are obtained from a wide variety of patients, including those with diseases affecting the sedimentation rate of the cellular components, indicates a need for continued improvement.
Analysis of the fluid dynamics of centrifugal separation of the components of physiological fluids such as blood has shown that the density of the red blood cell layer that develops during centrifugation is difficult to determine with precision due to the factors discussed above affecting the sedimentation rate of the red blood cells. The actual density of the red blood cell layer depends on the proportion of red blood cells and other components, principally plasma, in the layer. Thus, if a particular patient has any of a number of conditions that reduce the sedimentation rate of the cellular components, the actual density of the red blood cell layer will be less than expected because it will have a larger portion of plasma. In that instance, the actual position of a floating disc designed on the basis of an expected density of the red blood cell layer will not be the expected position.
Because the layer of the desired cells, such as platelets and bone marrow stem cells, after centrifugal separation is generally very thin, an error in the position of the separating element may affect the recovery rate significantly. That is, an error in the placement of the separating element on the order of the thickness of the layer of desired cells could result in differences in the recovery rate by as much as fifty percent.